1. Technical Field of the Invention
The present invention relates generally to the image signal processing field; and, more particularly, to a method and apparatus for improving image quality in digital cameras by adjusting color saturation according to the signal-to-noise ratio of raw image sensor output.
2. Description of Related Art
In a digital camera, such as a digital video or still camera, light from an object that is focused on an image sensing unit of the camera is converted into an electronic output image of the object that is suitable for display, transmission or storage. The image sensing unit converts the light pattern focused on it into an array of voltage samples that are converted to a digital signal. The digital signal is then processed by a digital image processor that renders the electronic output image. The digital image processor typically implements several different stages of image processing including, for example, image reconstruction from color mosaic samples, white-point adjustment, color correction, noise filtering, tone mapping and image compression.
In digital cameras, the digital image processor utilizes a color correction matrix for color correction. In most digital still cameras, the color correction matrix is determined in advance, and is usually selected to provide the most pleasing color reproduction under good lighting conditions.
In digital cameras also, the signal-to-noise ratio of the output of the image sensing unit decreases at low signal levels resulting in a noisy image, and the signal-to-noise ratio can be further degraded by the application of the color correction matrix resulting in an even noisier image.
Color correction matrices that have been used in known digital cameras are usually optimized for color quality at the expense of signal-to-noise ratio. In some of these cameras, the matrices decrease the signal-to-noise ratio by amplifying image noise; while in other cameras, noise amplification is avoided by switching to monochrome operation at low signal levels. In most known digital cameras, accordingly, images created at low signal levels tend to be either colorful and noisy, or colorless and less noisy.
U.S. Pat. No. 5,446,504 describes a method and apparatus for varying color saturation as a function of signal level in a digital camera. FIG. 2 of the patent illustrates a circuit that can be used to vary the color saturation over a full range from complete saturation to zero saturation, i.e., monochrome. In the method described in the patent, the color saturation is varied as a function of luminance level, and the saturation is decreased in dark areas of a scene.
A method such as described in U.S. Pat. No. 5,446,504 is not fully satisfactory for several reasons. Initially, in the method of the patent, the luminance signal controls the selection of the color correction matrix; however, the method operates only on relative luminance signal levels. In the method of the patent, accordingly, the color correction matrix must be varied throughout the image, depending on the relative luminance signal. Also, as indicated above, the electronic output image tends to be noisy or colorless at low signal levels.
What is needed is a digital imaging device, such as a digital camera, that has a color correction matrix that is capable of effectively correcting color over a wide range of signal levels.
The present invention provides a method and apparatus for improving image quality in a digital imaging device such as a digital camera., over a wide range of signal levels. A method for improving image quality according to the present invention comprises providing a digital image signal, and selecting a color corrector to correct the digital image signal. The color corrector is selected based, at least in part, on a signal-to-noise ratio of the digital image signal.
In the course of the present invention, it has been discovered that image quality can be optimized over a wide range of digital image signal levels in a digital imaging device, such as a digital camera, by correcting the color of the digital image signal utilizing information about the signal-to-noise ratio of the digital image signal.
In accordance with an embodiment of the present invention, the digital image signal comprises an output from an image sensing unit, and a signal-to-noise ratio estimator computes an estimate of the mean signal-to-noise ratio of the output digital image signal The color corrector, which may be a color correction matrix, is selected based on the estimated mean signal-to-noise ratio.
The estimate of the mean signal-to-noise ratio, according to a further embodiment of the invention, is computed from the mean level of the image sensing unit output, operating parameters of the image sensing unit and characteristics of the image sensing unit based on an a priori model of image sensing unit noise. The appropriate color corrector is then selected based on the computed estimate of the mean signal-to-noise ratio.
According to a further embodiment of the invention, the a priori model includes electronic noise sources ahead of and behind the gain stage of the image sensing unit, and photon shot noise. Other sources of noise, such as dark current could also be included in the model, if desired.
As mentioned previously, known digital cameras select a color correction matrix based on relative luminance signal levels. The degree to which the luminance signal has been amplified is unknown, and the characteristics and operating conditions of the image sensing unit are unknown. Consequently, it is not possible in known cameras to determine the signal-to-noise ratio of the digital image signal from the luminance level.
In accordance with an embodiment of the present invention, on the other hand, operating parameters are used to determine the absolute luminance signal level; and the signal-to-noise ratio is determined from the absolute signal level and characteristics of the image sensing unit. Because, in the present invention, the mean signal level is used to compute a mean signal-to-noise ratio; a single, selected color corrector can be used to correct the entire image, rather than having to vary the color corrector throughout the image. As a result, image quality of the digital image can be optimized over a wide range of signal values.
According to further embodiments of the invention, a look-up table can be used to implement the color corrector selection, or an analytical formula can be used to determine the color corrector by interpolating between two extreme values.
According to embodiments of the present invention, the image sensing unit comprises a CMOS (Complementary Metal Oxide Semiconductor) image sensor or a CCD image sensor; and the apparatus comprises a digital video or still camera. Furthermore, the invention provides embodiments with other features and advantages in addition to or in lieu of those discussed above. Many of these features and advantages are apparent hereinafter in conjunction with the following drawings and detailed description of exemplary embodiments of the invention.